Since an oxide superconducting material wherein RE2BaCuO5 phase is dispersed in a monocrystalline REBa2Cu3O7-x phase, where RE is a rare earth element, has a high critical current density (hereinafter sometimes referred to as “Jc”), it can be used as a superconducting bulk magnet which is excited by cooling in a magnetic field or the like to generate a strong magnetic field.
A superconducting bulk magnet has an excellent characteristic that a very strong magnetic field can be generated in a compact space. However, since the very strong magnetic field is confined in the compact space, a large electromagnetic stress will act inside the superconducting bulk body. This electromagnetic stress is also referred to as a hoop stress because it acts to spread the confined magnetic field. In the case of a strong magnetic field of 5 to 10 T class, the acting electromagnetic stress may exceed the mechanical strength of the material of the superconducting bulk body itself, and as a result, there is a possibility that the superconducting bulk body may be broken. If the superconducting bulk body is broken, the superconducting bulk body cannot generate a strong magnetic field.
If it is possible to prevent breakage of the superconducting bulk body, it is possible to take advantage of the superconducting bulk magnet which can be compact and can generate a strong magnetic field. The superconducting bulk magnet is expected to be useful in applications utilizing a magnet member for compact NMR (Nuclear Magnetic Resonance), or the drug delivery system using magnetic force, in order to achieve high performance of the resulting apparatus, as well as miniaturization and weight reduction of the resulting apparatus.
In order to prevent breakage of the superconducting bulk body by electromagnetic force, for example, Patent Document 1 proposes a superconducting bulk magnet composed of a columnar superconducting bulk body and a metal ring surrounding it. By adopting such a configuration, compressive stress due to the metal ring is applied to the superconducting bulk body when being cooled, and such a compressive stress has the effect of reducing the electromagnetic stress, so that cracking of the superconducting bulk body can be suppressed. Thus, Patent Document 1 shows that breakage of a columnar superconducting bulk body can be prevented.
Incidentally, in order to generate a high-strength magnetic field by magnetization using a monocrystalline oxide superconducting material having a general size (for example, a diameter of about 40 to 100 mm), it is effective to make each of the monocrystalline oxide superconducting material into a ring shape and to generate a strong magnetic field within the ring.
Generally, by machining a disk-like bulk material into a ring shape, it is possible to utilize a uniform magnetic field having a relatively high strength inside the ring. This makes it possible to apply to NMR or MRI (Magnetic Resonance Imaging), etc. where particularly high uniformity is required.
For example, Patent Document 2 discloses a superconducting magnetic field generating element obtained by combining seven hexagonal superconducting bulk bodies, disposing a reinforcing member made of a fiber reinforced resin or the like around the hexagonal superconducting bulk bodies, and further disposing a support member made of a metal such as stainless steel or aluminum alloy around the outer periphery thereof. Patent Document 3 discloses an oxide superconducting bulk magnet wherein a ring-shaped bulk superconducting bodies having a thickness in the c-axis direction of the crystal axis of 0.3 to 15 mm are laminated to each other. In addition, Patent Document 4 discloses a superconducting bulk magnet wherein a plurality of ring-shaped superconducting bodies of which their outer periphery and inner periphery are reinforced, are laminated. Further, Patent Document 5 discloses a superconducting bulk magnet wherein superconducting bodies having a multiple ring structure in the radial direction are laminated. In addition, Patent Document 6 discloses a bulk magnet wherein the outer periphery, the upper surface and the lower surface of one bulk body are reinforced. In addition, Patent Document 7 discloses a method for pulse magnetizing a bulk superconducting body by providing on the peripheral lateral surface of the bulk superconducting body, a heat radiating member which is a combination of an inner tube with a cut section, made of an annular aluminum alloy and an outer tube having no cut section, made of annular stainless steel arranged on the outer peripheral side of the inner tube. In the ring-shaped bulk superconducting body subject to the pulse magnetization, there is a risk to generate excessive eddy currents in the ring, causing heat generation. To prevent the excessive eddy currents, an inner tube made of an annular aluminum alloy is provided with the cut section.